Contractor-Ready Construction Documents–Part 1

To view the figures and tables associated with this article, please refer to the flipbook above.

Design decisions are typically based on previous experience, engineering judgment, and code requirements. However, the project location, general contractor, trade partners, and local practices may impact the structural design and detailing. Contractor involvement during design can provide input on key decisions in the selection of structural systems, use of building materials, and more. Instead of value engineering after the project has been completed, early contractor input can avoid costly redesign efforts and schedule delays. Part 1 will discuss contractor input that affects the design of the structure. Part 2 will discuss contractor input that affects detailing, constructability issues, and construction sequencing.

Concrete elevated floor systems can be constructed using either structural lightweight or normal weight concrete. Structural lightweight concrete can reduce an elevated slab’s weight and thickness. Lightweight concrete weighs approximately 107 to 112 pounds per cubic foot (pcf) versus approximately 145 pcf to 150 pcf for normal weight concrete. Lightweight concrete also provides better fire rating properties. For example, to achieve a two-hour fire floor rating, only 3¼ inches of structural lightweight concrete is required compared with 4½ inches of normal weight concrete (per Underwriters Laboratories UL assembly D-925). The lighter floor system decreases dead load, which reduces the beam, column, and foundation member sizes. However, structural lightweight concrete may not prove to be the most economical choice for a project. The availability and source of lightweight aggregates must first be discussed with the contractor. It may be more common for normal weight concrete to be specified in some locations, such as Alaska, Idaho, and others. The same holds true for lightweight concrete masonry units (CMU). While lightweight CMU is easier to handle during construction, its availability may be limited in some locales.

Design of concrete structures is not limited to just one solution. A range of member sizes and material strengths are available for each structural component. Understanding the availability and cost associated with high strength concrete may help determine member designs. For example, a spread footing or pile cap may be adequate with a 2-foot thickness and a 28-day compressive strength (f’c) = 8 kips per square inch (ksi), or a 4-foot thickness and f’c = 4 ksi. Either option may be adequate structurally, but the cost difference in mix design, material quantities, excavation depth, formwork, etc. may not be obvious without contractor input. As another example, specifying various concrete column compressive strengths at various columns within a given floor could result in mix design savings, but the contractor may prefer to use the higher strength concrete for all columns to avoid concrete mixer trucks with different strengths of concrete to maintain simplicity on site.

Another example is puddling at concrete columns which consists of placing higher strength concrete in the slab around a column before the rest of the slab is placed (Fig. 1). Discussion with the contractor may dictate if it is cheaper to puddle, increase the specified floor plate compressive strength, or increase the column size to reduce its required specified compressive strength.

There are more grades of reinforcement steel for concrete structures available now than ever before. High strength reinforcement, such as 75 ksi, 80 ksi, and 100 ksi are now options found in the American Concrete Institute’s ACI 318, Building Code Requirements for Structural Concrete, and the American Society for Testing and Materials (ASTM) specifications. Specifying higher grade reinforcement can help mitigate congestion issues, but their longer development lengths and material availability should be considered. Providing options for contractor review will allow for an economical design and minimize the risk of specifying high strength reinforcement that may not be available or would require a cost premium.

Architectural precast panels can provide strength, durability, and energy efficiency to the building facade. A wide variety of custom shapes and sizes are possible. Designing the structure for the weight of the precast panels successfully requires an understanding of the panel thickness, joint locations, and approximate locations of bearing/lateral connections. The floor heights, wind pressures, and building elevations will drive the panel thickness and connection locations. While it may be simplest to assume the precast panels can span between columns, bearing locations may be necessary on each side of the columns, or even at mid-span of the spandrel beams. The crane selection and site constraints may determine how the panels need to be broken up for erection purposes. Two possible precast loadings are shown in the sidebar. In one scenario the precast bearing connections are located two feet from the columns. In the second scenario, the precast panels are broken at mid-span adding additional point loads along the length of the beam. The total weight on the beam is the same, but the maximum moment and required moment of inertia are significantly different. Contractor and precast supplier involvement can provide input on the feasibility of complex precast shapes.

Design of the roof and floor framing may be affected by the need for the contractor to access the lower floors of the structure. Large items, e.g. construction materials or say prefabricated bathroom pods, may need to be dropped into place through the roof of the structure, which will require roof opening framing (Fig. 2). Similarly, if site constraints or crane reach limitations require the crane to be located within the building footprint, an opening at each floor and a required crane mat will affect the design of the structure and foundations.

Before proceeding with a proprietary system, the product should be reviewed with the contractor during the design phase. For example, SidePlate moment frame connections consisting of steel plates connected to the columns that are field bolted to plates and angles on the beams are widely used today. SidePlate eliminates complete penetration welds and ultrasonic testing. It is a best practice to ensure the contractor, fabricator, and erector are familiar with the proprietary system and licensing fee, and if not, get them connected.

Foundation types will vary between projects, but contractor input can assist in providing cost-effective and practical solutions.

For auger cast piles, geotechnical engineers will typically provide multiple pile diameter options (e.g. 14 inch, 16 inch, or 18 inch) and varying embedment depths. The contractor may have a preference on fewer piles with bigger diameters/deeper pile tip elevations compared with more piles with smaller diameters/shallower pile tip elevations. So for lightly loaded columns, is it better to switch pile diameters or keep the same pile diameter?

Drilled pier diameters could range from 24 inch diameter to over 120 inch. The pier diameter could vary from column to column with a consistent embedment depth, or the pier diameter could be kept constant and vary the embedment depth. There also could be a maximum pier diameter that is feasible for the drill rig; in lieu of using a 120 inch diameter drilled pier, it could be better to use two 84 inch diameter piers with a concrete grade beam to support the building column.

For additions adjacent to existing buildings, it is helpful to understand what drill rig will be used and how close a new drilled pier or auger cast pile can be installed next to an existing structure. If a building column with its drilled pier is located only a foot from an existing building, the drilled pier instead may be located say three feet away from the existing building with a concrete grade beam designed to cantilever over the drilled pier to support the building column.

Aggregate stone column foundations can be an effective intermediate foundation system and would allow for a high allowable bearing capacity (5 ksf to 8 ksf); however, the vibrations and noise from their installation should be considered. Sensitive equipment and occupant discomfort can be addressed with the contractor to determine if alternative ground improvement methods (e.g. rigid inclusions, compaction grouting) should be considered. Monitoring equipment may also be recommended for adjacent existing structures.

Due to expansive soils or undocumented fill on a site, the geotechnical engineer may recommend removing a significant volume of existing soils or using a framed first floor over a crawl space. Contractor pricing can provide insights to the owner on the most economical approach.

Contractor involvement during design can help take the guesswork out of what may be preferred and considered most economical by the contractor. The design can be optimized to provide a cost-effective project and minimize potential design and drawing changes after construction documents are issued. Part 2 of this series will discuss contractor input that affects detailing, constructability issues, and construction sequencing. ■

About the Authors

Jeremy Salmon, PE, SE, is a principal at Structural Design Group in Nashville, Tennessee. He may be reached at Jeremys@sdg-structure.com.

Zak Pruitt, PE, SE, is an associate principal at Structural Design Group and may be reached at zakp@sdg-structure.com.